Noninvasive detection of a certain analytes in human blood, or noninvasive blood test, has been a dream for the last two decades. It is convenient, hygienic, and comfortable. One way to do this is to use optical means, namely, Raman spectroscopy.
However, in such a noninvasive measurement, the Raman signal mainly arises from the skin and the spectral peaks corresponding to blood are not clear. While reflectance Raman signal is dominated by the response from the skin surface, transmission Raman signal contains more information from the bulk. Therefore a device which can capture both reflectance and transmission Raman signals is valuable for noninvasive detection of tissue and blood.
There have been many attempts to achieve both transmission Raman and reflectance Raman detection. Some may use one laser source and two detectors, or two laser sources and one detector or a combination of both. If only one laser and one detector is used, one has to add an optical combiner (for example fiber coupler) or to add an optical splitter (for example dichroic mirror); and in many instances, more than one additional optical components must be added. Each of these approaches has problems. In short, the problems are three folds. Firstly, the system with two detectors or two lasers is very much like two separate systems. Its bulky and expensive. Secondly, the system with additional optical components has additional background noise which is much larger than the blood signal we are going to detect. And lastly, to put all these optical components together the numerical aperture (e.g., the accepting angle of the collecting lens facing to the sample subject) becomes smaller. As a result, the sensitivity of the system becomes lower.
Therefore, a better approach is called for to capture transmission and reflectance Raman signals for physiological detections.